Semiconductor devices designed for high-frequency and high-voltage switching applications, particularly those designed for pulse-width controlled power conversion, are increasingly designed with compound semiconductor materials such as silicon carbide. These materials exhibit a wide bandgap between valence and conduction bands, and reasonably high mobility for free carriers. In addition, they can withstand a high internal electric field without breakdown, such as an electric field greater then 106 volts per centimeter (“cm”) that can be sustained in silicon carbide. The result is a device with excellent switching, voltage blocking, and conduction properties in high-power applications. Compared to a device as commonly formed with silicon, such compound semiconductor devices provide substantially improved performance in high-power circuits with only modest switching losses at relatively high switching frequencies.
However, high performance devices formed of compound semiconductors are often normally on devices. A normally on device is highly conductive when zero volts is applied to its control terminal, i.e., its gate. A normally on characteristic is generally difficult to apply in power switching applications due to the short circuit that can be produced when an end product is initially coupled to a power source.
A number of circuit arrangements, generally referred to as “cascode” circuits, are known to circumvent the short circuit that is created when an end product incorporating a normally on device is initially coupled to a power source. A cascode circuit also includes a silicon-based device that is normally “off” in series with the normally on device so that the series combination presents a normally off conduction condition when zero volts is applied to a control terminal. However, high-frequency switching characteristics of such cascode circuits are generally substantially inferior to those obtainable with a compound semiconductor device operating alone.
Thus, there is a need for a circuit arrangement including a normally on compound semiconductor device capable of blocking current flow when zero volts is applied to a control terminal that avoids the degradation of high-frequency switching performance that ordinarily occurs with the use of a normally off, cascode circuit arrangement.